Hitherto there are proposed a variety of structures for vibration-proof devices, e.g., a mount for supporting a vibrating object such as an automotive engine so as not to transmit its vibrations to a support body such as a car body.
For example, JP Utility Model Registration No. 2568225 discloses a hydraulic style vibration-proof device which comprises an upper attachment fitting to be attached to a vibrating object side and a cylindrical fitting to be attached to a support body side, both connected through a vibration-isolating base made of rubber elastomer, a diaphragm opposing the vibration-isolating base and situated on the lower side of the cylindrical fitting thus defining a liquid chamber internally enclosed between the vibration-isolating base and the diaphragm, a partition member having an orifice dividing the liquid chamber vertically, and a cup-shaped lower attachment fitting attached to a lower end opening of the cylindrical fitting thereby forming an air chamber between itself and the diaphragm.
At the lower attachment fitting, a bolt projecting downwardly is provided to install and fix it to the support body side. According to the aforementioned Reference, it is proposed to provide a bolt having a serration portion below its head so as to be pierced through the lower attachment part by press fitting and concurrently, with a view to ensuring air tightness of the air chamber, to fill the clearance gap between the head of the bolt and the lower attachment fitting and the clearance gap of the pierced part with an anaerobic dimethacrylate resin.
In this prior art vibration-proof device, for the lower attachment fitting attached to the cylindrical fitting, an iron press formed product is employed. That is, heretofore a serration bolt has been press fitted into the lower attachment fitting made of iron whereby the bolt has been seated firmly. Of late years, a low fuel consumption to automobiles is strongly demanded and accordingly, a request to weight reduction of automotive parts from automobile manufacturers is severer day by day. Under the circumstances, the vibration-proof device using the aforementioned iron-made lower attachment fitting is unable to meet sufficiently the request to weight reduction.
As a weight reducing means it is conceivable to replace iron of the lower attachment fitting with aluminum, but in that case if the structure of the iron-made lower attachment fitting is adopted directly for that of an aluminum-made one, then a problem will be caused in that the press fitted part of the bolt comes short of strength.
To cope with the problem, such a structure is conceivable that as shown in FIG. 6, an aluminum-made lower attachment fitting 100 is formed, at its bottom wall, with a female screw part 101 projecting upwardly and a stud bold 102 is embedded in the female screw part 101 or a normal bolt although not depicted, is used for fastening to the female screw part from the car body side. However, in this case, since the female screw part 101 is made of aluminum, it is necessitated to make its mating length to the bolt large for the purpose of ensuring the strength and consequently, the female part 101 projects largely within an air chamber 103, as a result of which a problem of interference to a diaphragm located above it arises.
Further such a structure is also conceivable that as shown in FIG. 7, an iron-made female screw part 111 is integrally formed at the bottom wall of an aluminum-made lower attachment part 110 by insert forming and a normal bolt 113 is used for fastening to the female screw part 111 from the car body 112 side or a stud bold is embedded in the female screw part, although not shown. However, the problem here is that the insert forming leads to an increase in cost.
In JP Patent Application Publication No. 1994-2735 A, a structure of aluminum-made lower attachment fitting is illustrated, wherein a bottom wall part thereof is partly thickened and a serration bolt is provided securely at the thick-walled portion so as to project downwardly. However, with that structure as disclosed in the Reference, the bottom wall part and a peripheral wall part of the lower attachment fitting are directly connected in the absence of any curving bent part smoothly linking between the bottom wall part and the peripheral wall part, although the corner parts are formed in a somewhat rounded form. Moreover the thickness from the bottom wall part to the peripheral wall part is gradually decreased only and a sufficient thickness at the boundary of the bottom wall part and the peripheral wall part is not necessarily secured. As a consequence, stress generated by excessive load input to the vibration-proof device cannot be effectively absorbed and accordingly, a problem in the aspect of strength is worried about